Don't think that it would make much of a difference, except on brush wear. Building lather isn't about friction but about aeration of the 'solution'. In that respect, the brush (besides being a water reservoir) act like a whisk. Cream doesn't whip easier in a rough bowl either.
The only advantage of a porous bowl that I can think of would be the additional infusion of the lather with air (from the pores) from below, but I can also think of a few properties of a porous bowl that would be deleterious to you lather, so in all, I would guess (and note that at this point, everything said above is conjecture, none is hard solid, experiment-supported fact...) that the smooth bowl is actually the best implement to build your lather in...
Henk